1. Field of the Invention
The present invention relates generally to mass flow rate and density measuring apparatus and more particularly to an improved means for measuring the mass flow rate of a flowing mass using the effects of Coriolis forces and centrifugal forces upon an oscillatorially translated or deflected portion of one or more loops of conduit through which the mass flow is caused to pass.
2. Description of the Prior Art
There has been a continuing need for more accurate and more efficient devices for determining the mass flow rate and density of fluids and flowing solids passed through pipe lines and other various types of conduit. Prior art flow meters of the type including the present invention have in the past been embodied as gyroscopic mass flow meters or Coriolis type mass flow meters and the like.
One such device which utilizes Coriolis forces to measure mass flow is disclosed in U.S. Pat. No. 4,109,524 entitled "Method and Apparatus for Mass Flow Rate Measurement", issued Aug. 29, 1978 to James E. Smith. In this patent an apparatus is disclosed wherein a mechanically reciprocating force is applied to first and second sections of a straight conduit by means of a beam that is disposed parallel to the first and second sections and has its ends mechanically linked to the adjacent ends of the two conduit sections. The adjacent ends of the first and second conduit sections are connected together by means of a short segment of conduit and flexible couplings and the opposite ends of each conduit section is separately supported in cantilever fashion relative to a base structure. The reciprocating forces applied to the conduit are resisted by separate Coriolis forces developed in the first and second conduit sections which act in opposite directions and induce a force moment about the center of the beam which is measured by a torque sensor. By measuring the force moment induced in the conduits (and transferred to the beam) by the Coriolis reactant forces, measurement of the mass flow through the conduit may be made. However, the measurement is complicated because of the need to avoid spurious measurements of the forces resulting from seismic or other vibrational forces transmitted through the support structure. Other similar devices are disclosed in the U.S. patents to Wiley et al, U.S. Pat. Nos. 3,080,750; Sipin, 3,218,851; Souriau, 3,396,579; and Sipin 3,329,019.
Rather than use linear sections of conduit that are pivoted at opposite ends and reciprocated at the adjacent ends, a U-tube or similar configuration is more commonly employed in mass flow measurement. In such cases, the inlet and outlet ends of the legs of the U-shaped tube are fixedly mounted to a base and the bight end of the U-tube is reciprocated. The differential displacement of corresponding portions of the U-tube side legs caused by Coriolis influence on the flow is then measured as an indicator of mass flow rate. Such a technique and apparatus is suggested in the above-mentioned Smith patent and is illustrated in U.S. Pat. No. 4,187,721 for "Method and Structure for Flow Measurement" issued Feb. 12, 1980 to James E. Smith, now U.S. Pat. No. Re. 31,450. As disclosed in the referenced patent, a U-shaped conduit is mounted in a cantilevered manner at the leg ends thereof, and an oscillating means is mounted on a spring arm having a natural frequency substantially equal to that of the U-shaped conduit and is used to provide up and down motion to the center of the bight end thereof. Measuring sensors (flags and photodetectors) are provided which detect the leading and trailing portions of the legs of the U-shaped conduit as they pass through a plane defined by the U-shaped conduit at substantially the mid-point of its oscillation. The time differential of passage of the legs through the midplane is measured as an indication of mass flow rate. Essentially, the same structure is used in the subsequent Smith U.S. Pat. No. 4,422,338 referenced below except that in the latter, a pair of velocity sensors are substituted for the photodetectors, and conditioning electronics are provided for developing signals corresponding to the passage of the side legs through the midplane.
In U.S. Pat. No. 4,127,028 entitled "Coriolis Mass Flow Rate Metering Means" issued Nov. 28, 1978 to Bruce M. Cox, et al, a pair of vibrating generally U-shaped tubes are fixedly mounted at the inlet and outlet ends thereof, in parallelly disposed, spaced apart cantilevered fashion so that the bight ends of the respective tubes are free to move relative to each other. An oscillatory drive mechanism is connected between the bight ends of the respective tubes and actuated to provide opposing reciprocation thereof such that the U-shape members act as the tines of a tuning fork. The frequency of the oscillation of the tubes is adjusted until the tubes vibrate a fixed displacement when a known material is flowing therethrough. The power needed to vibrate the tubes the known displacement at a fixed frequency determines the density of an unknown fluent material flowing the U-shaped tubes. Mass flow rate is detected by photodetectors positioned to operate in the same manner as taught by Smith for a single tube embodiment. Cox also suggests that strain gages or velocity sensors could be substituted for the photodetectors, and acknowledges that it is known in the prior art that there will be a phase shift between the outputs of the two sensors which is proportional to the Coriolis force couple.
The principle teaching of this reference is the narrowing of the separation of the legs of each U-shaped tube proximate the support ends thereof so as to improve the freedom of torsional twist that may be imposed upon the respective tubes by the Coriolis reactance forces. This reference also illustrates a looped tube configuration in FIG. 5 thereof, but fails to teach or suggest how such configuration might be used to provide enhanced flow measurement. It is therefore not believed to anticipate the present invention.
Other prior art known to the present inventor may be found in the U.S. Pat. Nos. to Barnaby et al, 2,752,173; Roth 2,865,201 and 3,049,919; Sipin, 3,355,944; Sipin 3,485,098; Catherall 3,955,401; and Shiota 4,381,680, and the EPO application of Smith, Publication No. EP 0 083 144 A1 which corresponds to U.S. Pat. No. 4,422,338. A listing of prior art utilizing the Coriolis principle may be found in the above-referenced Sipin U.S. Pat. No. Re. 31,450.
A disadvantage of the Smith and Cox type of flow measuring devices, as well as those of others in the prior art, is that they are highly sensitive to to external vibrations which cause the measuring tube or tubes to be subjected to twisting forces other than those imparted by Coriolis reaction forces, and such forces interfere with the accurate measurement of mass flow.
Another disadvantage pertaining to the preferred embodiments in the Smith Reissue U.S. Pat. No. Re. 31,450 and Smith U.S. Pat. No. 4,422,338 is that the proposed methods of time differential measurement at the midplane of the U-tube will produce flow measurement errors when the fluid density is changing.
Yet another disadvantage of the prior art Coriolis type devices is that they are not capable of providing accurate flow data over a wide range of flow due to limitations in sensitivity in the flow structure used.
Still another disadvantage of the prior art devices is that they are not provided with dynamic damping means to reduce the sensitivity to external vibrations.
Yet another disadvantage of the prior art Coriolis type devices is that they utilize a directly proportional relationship between mass flow rate and differential phase angle or differential time measurements.
A still further disadvantage of the prior art Coriolis type devices is that they have substantial errors in mass flow rate if the temperature of the sensing structure changes.
Still another disadvantage of the prior art described in the Smith U.S. Pat. No. Re. 31,450 and U.S. Pat. No. 4,422,338 and Cox, Gonzales U.S. Pat. No. 4,127,028 is that the oscillatory drive motion creates significant bending stress especially at the points where the U-tubes are attached leading to significant danger of stress corrosion. In accordance with the present invention, the drive motion causes primarily torsional stresses absorbed over a certain distance, such as 38 in FIG. 2, resulting in much lower danger to stress corrosion thereby enhancing safety.
Still another disadvantage of the prior art shown in the above patents is that they introduce at least two sharp 90 degree bends in the flow patterns, leading to high pressure drop. The present invention uses loops which have a gradual curvature or circle which gives lower pressure drop.